Most frequent points defects in as grown cubo-octahedral high-pressure and high-temperature (HPHT) diamond crystal are the following:                vacancy (V), a defect caused by an atom missing from a lattice site;        interstitial, when an extra atom is introduced in the structure at a position between normally occupied lattice sites, for instance an interstitial nitrogen atom (NI).        substitutional, which involves the replacement of an atom of a specific type by an atom of a different type, for instance, isolated/single substitutional nitrogen atom (NS) replacing a carbon atom.        
Nitrogen-Vacancy (N-V) centres are formed by combining NS with V. The N-V centre absorbs excitation light in a wavelength range of 480 to 638 nm and emits luminescence in a wavelength range of 638 to 780 nm. To form the N-V centre in the diamond, a rough diamond containing NS is irradiated by an electron beam having an energy of few MeV, typically less than 4 MeV, to generate Lattice defects. Then, the irradiated diamond is annealed to combine the lattice defect with the nitrogen atom to form the N-V centre. During annealing, the vacancies move by a thermally-activated jump from one lattice site to the next. The probability of making this jump is v exp[−Ea/(kBT)] where v is the “attempt-to-escape frequency” and Ea is the activation energy. In a type Ib (i.e. discrete dispersion type) diamond, this random walk continues until a vacancy V encounters a isolated nitrogen atom NS, where it is trapped to form a N-V centre. There are limitations to N-V centre formation and uniformity that can be produced as a consequence of competitive defect formation and because of the strong growth sector dependence associated with the concentration of defects such as nitrogen in diamond.
The N-V centre in diamond is one of those defect studied in most detail. It has C3v symmetry, with the symmetry axis oriented along the crystallographic [111] axis. The main photo physical parameters of the N-V centre indicate the suitability of the system for single centre detection; they have a large absorption cross section at the excitation wavelength, a short excited-state lifetime, and high quantum efficiency for radiative relaxation. In addition, no efficient shelving in a metastable state has been reported for N-V centres at room temperature, although the high spectral hole-burning efficiency at low temperature indicates existence of this process. This colour centre has the great advantage of being photostable and do not exhibit any photoblinking when exposed to a laser at 532 nm, with a typical intensity in the range of few mW cm−2. Untreated samples of synthetic Ib diamond provide a concentration of N-V centers well suited for addressing individual centres.
The U.S. Pat. No. 4,880,613 discloses a light emitting element comprising diamond which contains N-V centres and optionally H3 colour centres (N-V-N). The disclosed method to manufacture such diamond implies an electron beam of energy of 2 to 4 MeV and a dose of 1×1017 to 2×1018 electrons*cm−2 for generation of N-V centres. With such acceleration energy, the electron beam is not efficient when the thickness of diamond is above 3 or 4 millimeters. Therefore, document U.S. Pat. No. 4,880,613 suggests to use a neutron beam when the thickness of diamond is larger. It means that the quantity of diamonds that can be irradiated by an electron beam in a single batch is limited to the volume defined by the scanning area of the electron beam and a depth ≦3 mm.
A significant limitation in bulk diamond is that the light is emitted in a high index material, that makes its efficient extraction difficult. Refraction at the sample interface leads to a small collection solid angle and to aberrations. The subwavelength size of nanocrystals renders refraction irrelevant. A nanocrystal may be thought of as a point source emitting light in air. Gruber et al., “Scanning Confocal Optical Microscopy and Magnetic Resonance on Single Defect Centres”, Science 276, 2012-2014, 1997 is the first paper to describe N-V centres in diamond nanocrystals using confocal microscopy, magnetic resonance, photoluminescence, etc.
A promising system for a robust single photon source is provided by individual nitrogen-vacancy colour centres (N-V centres) in diamond. Light sources able to emit individual photons on demand would be of great potential use for quantum cryptography. A quantum computation scheme requiring such sources has also been proposed recently. Considerable activity is thus dedicated to designing and implementing efficient, robust, room-temperature sources delivering a periodic train of pulses containing one and only one photon. These sources are based on the property of a single emitting dipole to emit only one photon at a time. When excited by a short and intense pulse, such an emitter delivers one and only one photon. After pioneering experiments have demonstrated photon antibunching and conditional preparation of single-photon states, followed by first attempts to build triggered single photon sources, the present generation of experiments is concentrating on solid-state schemes better suited for practical use, such as single organic molecules, self-assembled semiconductor quantum dots, or semiconductor nanocrystals. The successful candidate should work at room temperature, and be photostable. In this framework, for instance, Beveratos et al, “Nonclassical radiation from diamond nanocrystals”, Phys. Rev. A 061802, 1-4. (2001) discloses a preparation of nanocrystals from synthetic diamond powder bought from de Beers. The defects were created by irradiation with 1.5 MeV beam at a dose of 3×1017 cm−2 electrons, and annealing in vacuum at 850° C. during 2 hours. The nanocrystals were dispersed by sonification in a solution of polymer Polyvinylpyrrolidone at 1 wt. % in propanol. This allows the disaggregation of the particles and their stabilization in a colloidal state. Centrifugation at 11 000 rpm for 30 mn allows the authors to select nanocrystal sizes of 90±30 nm measured by dynamical light scattering. The average number of N-V centres in a nanocrystal has been evaluated to 8. The density of N-V centres created is then estimated to be of one in a theoretical 30 nm diameter sphere.
Biological fluorescent probe, such as organic dyes, fluorescent proteins and fluorescent semiconductor nanocrystals (or quantum dots), are able to absorb light at a wavelength longer than 500 nm and emit light at a wavelength longer than 600 nm, at which the emission has a long penetration depth through cells and tissues. These probes have several detrimental properties, such as photobleaching and blinking or cytoxicity and chemical instability (complicated quantum dot surface chemistry). On the other hand, it has been shown recently that diamond nanocrystals bearing intracrystalline N-V centre, which are known to absorb strongly at around 560 nm and emits fluorescence efficiently at around 700 nm, which is well separated from the spectral region where endogenous fluorescence occurs, are nontoxic, and allow long-term monitoring of a single diamond nanoparticle in a biological cell. Yu et al., “Bright fluorescent nanodiamonds: no photobleaching and low cytotoxicity”, J Am Chem Soc. 21, 17604-5 (2005) produced highly fluorescent nanodiamonds (FND) with low cytoxicity from synthetic type Ib diamond powders with a nominal size of 100 nm by irradiation with a 3 MeV proton beam at a dose of 5×1015 ions/cm2 and subsequent annealing at 800° C. in vacuum for 2 h. Wei P. and al., “Characterization and application of single fluorescent nanodiamonds as cellular biomarkers”, PNAS 104, 727-732, (2007) prepares fluorescent diamond nanoparticles from synthetic Ib type diamond particles of 35 or 100 nm by proton irradiation (3 Mev, 1016 cm−2 ions), subsequent annealing in vacuum at 700° C. for 2 h, removal of non-diamond shell and surface functionalization with carboxylic or amino groups. The authors showed that the fluorescence of a single 35 nm diamond is significantly brighter than that of a single dye molecule such as Alexa Fluor 546. The latter photobleached in the range of 10 s at a laser power density of 104 W cm−2, whereas the nanodiamond particle showed no sign of photobleaching even after 5 min of continuous excitation. Furthermore, no fluorescence blinking was detected within a time resolution of 1 ms. The photophysical properties of the particles did not deteriorate even after surface functionalization with carboxyl groups, which form covalent bonding with polyL-lysines that interacts with DNA molecules through electrostatic forces. The feasibility of using surface-functionalized fluorescent nanodiamonds as single-particle biomarkers was demonstrated with both fixed and live HeLa cells.